1. Field of the Invention
This invention relates to methods of densifying foamed plastic material and, more particularly, to a method of densifying a mass of open-celled polyether or polyester polyurethane foam. While the method is advantageous for material of any thickness, it is especially valuable in treating material which is several inches thick.
2. Description of the Prior Art
Previous methods of densifying open-celled plastic foam articles have relied on the extended application of heat and substantial compressive forces to the surface of the article. This is often carried out by means of large heated metal molds or platens between which the foam is placed. Heat is transmitted to the foam by conduction from the heated platens for a substantial period of time. The low K-factors of air and of polyurethane make such heating processes very time-consuming.
A commercially acceptable polyurethane foam requires a density and other physical characteristics which are quite uniform throughout. This can only be achieved by bringing all parts of a mass of such foam into a predetermined temperature range during processing because the material will return to its pre-treatment state if the treatment temperature is too low, and this necessarily causes a non-uniform product if some parts of the mass are sufficiently hot during treatment and others are not. Prior art processes inherently tend to produce non-uniform material, and are difficult to control because of the very low K-factors previously mentioned.
U.S. Pat. No. 3,577,519 (Gambardella) describes a method of densifying polyurethane foam buns wherein partially cured foam is compressed while still at or near its exothermal reaction temperature. This process may result in non-uniform temperatures and uncontrollable densification, at least in part, due to the relatively rapid cooling of the exterior of the mass and the typically large size of polyurethane reaction buns. This problem is only patially alleviated by the use of heated compression means. Also, the process may not be used to densify fully cured foam.
Further, the heating of a mass of foam by conduction, as in the above-described prior processes, is very inefficient, as great amounts of heat are required to initially heat the compression means, which are often massive, and then to maintain the temperature of the compression means while they are in prolonged contact with the foam.
Another source of inefficiency in previous compression methods is the inability of those methods to adapt to continuous operating conditions. The nature of the prior processes eliminates anything but extremely thin material for densification on a continuous system. Prior processes were generally operated on an intermittent basis which would result in platen marks on continuous strips of foam.
Another disadvantage of prior compression densification methods as applied to cured polyurethane is that very high compressive forces are required until the mass reaches the proper temperature. This requires the use of massive compressive equipment, the cost of which further detracts from the desirability of prior compression methods.
U.S. Pat. No. 3,475,525 (Peters) describes a method of reticulating open-celled polyurethane foam in which a stream of heated gas is passed through the foam in order to heat the foam so as to melt or thermally decompose membranous cell walls without melting the network strands supporting the cell walls. The gas temperature must be above about 480.degree. F. and is generally between about 750.degree. F. and 1470.degree. F., and the thickness of the foam must be less than 0.75 inches to prevent degradation of the network structure. Additionally, the gas velocity through the foam must be in excess of about 31/4 ft./sec.
The method of the present invention preserves the cell integrity, since gas temperature and velocity are substantially less than in Peters. Further, the present invention may be effectively applied to foams of thickness much greater than 0.75 inches, the upper limit of the Peters method.